1. Field of the Invention
This invention relates generally to the field of electric current sensing devices. More particularly, this invention relates to a device and method for sensing electric current flowing in a conductor by concentrating magnetic flux of the magnetic field associated with the current and detecting that flux with a Hall Effect transducer.
2. Description of the Prior Art
There are several reasons why it is desirable to be able to sense and measure the electric current flowing in a service conductor for industrial plants, businesses or homes. One reason is to permit monitoring the load in the electrical system that is served by the conductor. A second is to both monitor and regulate power consumption so as to take advantage of reduced power rates charged by utility companies during certain time intervals, and to limit usage during high rate times. Regulation of consumption in an electrical system is especially advantageous when the user rate set by the utility companies increases sharply once a certain level of power consumption is reached. If the load in an electrical system can be reduced or partially shed when the critical level of power consumption is reached, the user can economically optimize its power use. The corresponding advantage to utility companies is that peak power loads can be reduced.
There exist load management systems in the prior art that are activated by devices that indicate the instantaneous level of power consumption by the electrical system.
One known load sensing device is a pulse counter for counting pulses from a wattmeter installed by a utility company, which often generates pulses at a rate proportional to power consumption. However, not all wattmeters are of the type which generate such pulses and such counters therefore have limited application.
A second known device is a current transformer, which is used to tap a small, predetermined fraction of a large current to be measured, permitting measurement by a small ammeter or wattmeter. In one prior art arrangement, current transformers are used together with Hall Effect transducers. (See U.S. Pat. No. 4,034,233) (These transducers can be used to sense the alternating magnetic field that is associated with an alternating electric current flowing in a conductor. Because that magnetic field has a strength and polarity that is determined by and proportional to the current, a Hall Effect transducer is, in effect, sensing the strength of the current tapped by the current transformer.) Thus, in the known device, the Hall Effect transducer voltage output is proportional to a sensed magnetic field strength, which is proportionally related to the current in the current transformer, which, in turn, is proportional to the current in the line to which the transformer is coupled. By knowing the characteristics of the current transformer and the transducer and by application of the well known relationship between voltage and current, the power level may be determined.
A primary disadvantage of a current/power sensing device using a current transformer is its size. In homes and smaller commercial buildings the electrical panels are not large and there may not be a great deal of available space between the point at which the power feeder lines enter the panel and the bus where they are connected. Accordingly, when it is desired to insert a current sensing device at the electrical panel, use of a current transformer may be very difficult or impossible unless a larger panel is installed or a junction box is placed in the conduit system upstream from the electrical panel. In either case, a technician may be required and several hours labor may be necessary. In addition, unless a split current transformer is used (which is usually larger and more expensive) the power feeder lines may have to be disconnected to install the current transformer.
In some applications, a Hall Effect device simply placed in proximity to a current-carrying conductor can be used. The disadvantages of such an arrangement are its limited sensitivity and possible inaccuracy when magnetic conductors approach the current conductor and influence the strength of the field to be measured. It is also known that sensitivity of a Hall Effect current sensing device can be improved by use of a toroidal core. "Hall Effect Transducers", a 1982 publication of the Micro Switch division of Honeywell shows use of a toroidal core having an air gap in which a Hall Effect device is inserted. The conductor in which current is to be sensed passes through the center of the toroid or is wrapped around the toroid for one or more turns.